Build: Update ESLint to v9, migrate to flat config, lint dist files

Dist files linting is limited to checking if they're proper ES5.

Closes gh-2336

16 files changed, 181 insertions, 100 deletions

diff --git a/.eslintignore b/.eslintignore
deleted file mode 100644
index 5e992599f..000000000
--- a/.eslintignore
+++ /dev/null
@@ -1,4 +0,0 @@
-dist/**/*
-external/**/*
-tests/lib/vendor/**/*
-ui/vendor/**/*
diff --git a/.eslintrc.json b/.eslintrc.json
deleted file mode 100644
index e7d67eb0e..000000000
--- a/.eslintrc.json
+++ /dev/null
@@ -1,21 +0,0 @@
-{
-  "root": true,
-
-  "extends": "jquery",
-
-  // Uncomment to find useless comment disable directives
-  // "reportUnusedDisableDirectives": true,
-
-  "parserOptions": {
-    "ecmaVersion": 2018
-  },
-
-  "env": {
-    "es6": true,
-    "node": true
-  },
-
-  "rules": {
-    "strict": [ "error", "global" ]
-  }
-}
diff --git a/.github/workflows/node.js.yml b/.github/workflows/node.js.yml
index 6279223a3..eef7c86e7 100644
--- a/.github/workflows/node.js.yml
+++ b/.github/workflows/node.js.yml
@@ -49,12 +49,13 @@ jobs:
       - name: Install npm dependencies
         run: npm install
 
-      - name: Lint
-        run: npm run lint
-
       - name: Build
         run: npm run build
 
+      # Lint must happen after build as we lint generated files.
+      - name: Lint
+        run: npm run lint
+
       - name: Test
         run: |
           npm run test:unit -- \
diff --git a/Gruntfile.js b/Gruntfile.js
index cc532b6e9..bbb71d33e 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -204,9 +204,12 @@ grunt.initConfig( {
 			"ui/**/*.js",
 			"!ui/vendor/**/*.js",
 			"Gruntfile.js",
+			"dist/jquery-ui.js",
+			"dist/jquery-ui.min.js",
 			"build/**/*.js",
 			"tests/unit/**/*.js",
 			"tests/lib/**/*.js",
+			"!tests/lib/vendor/**/*.js",
 			"demos/**/*.js"
 		]
 	},
@@ -401,7 +404,7 @@ grunt.registerTask( "lint", [
 	"htmllint"
 ] );
 grunt.registerTask( "build", [ "requirejs", "concat", "minify:main" ] );
-grunt.registerTask( "default", [ "lint", "build" ] );
+grunt.registerTask( "default", [ "build", "lint" ] );
 grunt.registerTask( "sizer", [ "requirejs:js", "minify:main", "compare_size:all" ] );
 grunt.registerTask( "sizer_all", [ "requirejs:js", "minify", "compare_size" ] );
 
diff --git a/demos/.eslintrc.json b/demos/.eslintrc.json
deleted file mode 100644
index 805ec8eb2..000000000
--- a/demos/.eslintrc.json
+++ /dev/null
@@ -1,9 +0,0 @@
-{
-	"root": true,
-
-	"extends": "../ui/.eslintrc.json",
-
-	"globals": {
-		"require": true
-	}
-}
diff --git a/eslint.config.mjs b/eslint.config.mjs
new file mode 100644
index 000000000..4fb03f6b3
--- /dev/null
+++ b/eslint.config.mjs
@@ -0,0 +1,162 @@
+import jqueryConfig from "eslint-config-jquery";
+import globals from "globals";
+
+export default [
+	{
+		ignores: [
+			"dist/**/*",
+			"!dist/jquery-ui.js",
+			"!dist/jquery-ui.min.js",
+			"external/**/*",
+			"tests/lib/vendor/**/*",
+			"ui/vendor/**/*"
+		]
+	},
+
+	{
+		ignores: [ "dist/**/*" ],
+		rules: {
+			...jqueryConfig.rules,
+			"no-unused-vars": [
+				"error",
+				{
+					argsIgnorePattern: "^_",
+					caughtErrorsIgnorePattern: "^_"
+				}
+			]
+		}
+	},
+
+	{
+		files: [ "Gruntfile.js" ],
+		languageOptions: {
+			ecmaVersion: "latest",
+			sourceType: "commonjs",
+			globals: {
+				...globals.node
+			}
+		},
+		rules: {
+			strict: [ "error", "global" ]
+		}
+	},
+
+	{
+		files: [ "eslint.config.mjs" ],
+		languageOptions: {
+			ecmaVersion: "latest",
+			sourceType: "module",
+			globals: {
+				...globals.node
+			}
+		},
+		rules: {
+			strict: [ "error", "global" ]
+		}
+	},
+
+	// Source, demos
+	{
+		files: [ "ui/**/*.js", "demos/**/*.js" ],
+		languageOptions: {
+			ecmaVersion: 5,
+			sourceType: "script",
+			globals: {
+				...globals.browser,
+				...globals.jquery,
+				define: false,
+				Globalize: false
+			}
+		},
+		rules: {
+			strict: [ "error", "function" ],
+
+			// The following rule is relaxed due to too many violations:
+			"no-unused-vars": [
+				"error",
+				{
+					args: "after-used",
+					argsIgnorePattern: "^_",
+					caughtErrorsIgnorePattern: "^_"
+				}
+			],
+
+			// Too many violations:
+			camelcase: "off",
+			"no-nested-ternary": "off"
+		}
+	},
+	{
+		files: [ "ui/i18n/**/*.js" ],
+		rules: {
+
+			// We want to keep all the strings in separate single lines
+			"max-len": "off"
+		}
+	},
+
+	// Dist files
+	// For dist files, we don't include any jQuery rules on purpose.
+	// We just want to make sure the files are correct ES5.
+	{
+		files: [ "dist/jquery-ui.js", "dist/jquery-ui.min.js" ],
+		languageOptions: {
+			ecmaVersion: 5,
+			sourceType: "script"
+		},
+		linterOptions: {
+			reportUnusedDisableDirectives: "off"
+		}
+	},
+
+	// Build
+	{
+		files: [ "build/**/*.js" ],
+		languageOptions: {
+			ecmaVersion: "latest",
+			sourceType: "commonjs",
+			globals: {
+				...globals.node
+			}
+		},
+		rules: {
+			"no-implicit-globals": "error",
+			strict: [ "error", "global" ]
+		}
+	},
+
+	// Demos
+	{
+		files: [ "demos/**/*.js" ],
+		languageOptions: {
+			globals: {
+				require: true
+			}
+		}
+	},
+
+	// Tests
+	{
+		files: [ "tests/**/*.js" ],
+		languageOptions: {
+			ecmaVersion: 5,
+			sourceType: "script",
+			globals: {
+				...globals.browser,
+				...globals.jquery,
+				define: false,
+				Globalize: false,
+				QUnit: false,
+				require: true,
+				requirejs: true
+			}
+		},
+		"rules": {
+
+			// Too many violations:
+			"max-len": "off",
+			"no-unused-vars": "off",
+			strict: "off" // ideally, `[ "error", "function" ]`
+		}
+	}
+];
diff --git a/package.json b/package.json
index c34bef799..00f9e07fe 100644
--- a/package.json
+++ b/package.json
@@ -58,13 +58,14 @@
 		"@swc/core": "1.11.5",
 		"commitplease": "3.2.0",
 		"eslint-config-jquery": "3.0.2",
+		"globals": "16.0.0",
 		"grunt": "1.6.1",
 		"grunt-bowercopy": "1.2.5",
 		"grunt-compare-size": "0.4.2",
 		"grunt-contrib-concat": "2.1.0",
 		"grunt-contrib-csslint": "2.0.0",
 		"grunt-contrib-requirejs": "1.0.0",
-		"grunt-eslint": "24.0.1",
+		"grunt-eslint": "25.0.0",
 		"grunt-git-authors": "3.2.0",
 		"grunt-html": "17.1.0",
 		"jquery-test-runner": "0.2.5",
diff --git a/ui/.eslintrc.json b/ui/.eslintrc.json
deleted file mode 100644
index eaba81af2..000000000
--- a/ui/.eslintrc.json
+++ /dev/null
@@ -1,42 +0,0 @@
-{
-	"root": true,
-
-	"extends": "jquery",
-
-	"parserOptions": {
-		"ecmaVersion": 5
-	},
-
-	"env": {
-		"browser": true,
-		"jquery": true,
-		"node": false
-	},
-
-	"rules": {
-		"strict": [ "error", "function" ],
-
-		// The following rule is relaxed due to too many violations:
-		"no-unused-vars": [ "error", { "vars": "all", "args": "after-used" } ],
-
-		// Too many violations:
-		"camelcase": "off",
-		"no-nested-ternary": "off"
-	},
-
-	"globals": {
-		"define": false,
-		"Globalize": false
-	},
-
-	"overrides": [
-		{
-			"files": [ "i18n/**/*.js" ],
-			"rules": {
-
-				// We want to keep all the strings in separate single lines
-				"max-len": "off"
-			}
-		}
-	]
-}
diff --git a/ui/effect.js b/ui/effect.js
index bbbb733c3..cb9ab8043 100644
--- a/ui/effect.js
+++ b/ui/effect.js
@@ -9,9 +9,7 @@
 
 //>>label: Effects Core
 //>>group: Effects
-/* eslint-disable max-len */
 //>>description: Extends the internal jQuery effects. Includes morphing and easing. Required by all other effects.
-/* eslint-enable max-len */
 //>>docs: https://api.jqueryui.com/category/effects-core/
 //>>demos: https://jqueryui.com/effect/
 
@@ -320,7 +318,7 @@ if ( $.uiBackCompat === true ) {
 			try {
 				// eslint-disable-next-line no-unused-expressions
 				active.id;
-			} catch ( e ) {
+			} catch ( _e ) {
 				active = document.body;
 			}
 
diff --git a/ui/effects/effect-explode.js b/ui/effects/effect-explode.js
index ed40833a8..da38b4d55 100644
--- a/ui/effects/effect-explode.js
+++ b/ui/effects/effect-explode.js
@@ -9,9 +9,7 @@
 
 //>>label: Explode Effect
 //>>group: Effects
-/* eslint-disable max-len */
 //>>description: Explodes an element in all directions into n pieces. Implodes an element to its original wholeness.
-/* eslint-enable max-len */
 //>>docs: https://api.jqueryui.com/explode-effect/
 //>>demos: https://jqueryui.com/effect/
 
diff --git a/ui/widgets/accordion.js b/ui/widgets/accordion.js
index ff6e4631d..43a50db83 100644
--- a/ui/widgets/accordion.js
+++ b/ui/widgets/accordion.js
@@ -9,9 +9,7 @@
 
 //>>label: Accordion
 //>>group: Widgets
-/* eslint-disable max-len */
 //>>description: Displays collapsible content panels for presenting information in a limited amount of space.
-/* eslint-enable max-len */
 //>>docs: https://api.jqueryui.com/accordion/
 //>>demos: https://jqueryui.com/accordion/
 //>>css.structure: ../../themes/base/core.css
diff --git a/ui/widgets/datepicker.js b/ui/widgets/datepicker.js
index 323723b89..029f255e8 100644
--- a/ui/widgets/datepicker.js
+++ b/ui/widgets/datepicker.js
@@ -1,4 +1,4 @@
-/* eslint-disable max-len, camelcase */
+/* eslint-disable max-len */
 /*!
  * jQuery UI Datepicker @VERSION
  * https://jqueryui.com
@@ -535,7 +535,7 @@ $.extend( Datepicker.prototype, {
 	_getInst: function( target ) {
 		try {
 			return $.data( target, "datepicker" );
-		} catch ( err ) {
+		} catch ( _err ) {
 			throw "Missing instance data for this datepicker";
 		}
 	},
@@ -768,7 +768,7 @@ $.extend( Datepicker.prototype, {
 					$.datepicker._updateAlternate( inst );
 					$.datepicker._updateDatepicker( inst );
 				}
-			} catch ( err ) {
+			} catch ( _err ) {
 			}
 		}
 		return true;
@@ -1540,7 +1540,7 @@ $.extend( Datepicker.prototype, {
 
 		try {
 			date = this.parseDate( dateFormat, dates, settings ) || defaultDate;
-		} catch ( event ) {
+		} catch ( _err ) {
 			dates = ( noDefault ? "" : dates );
 		}
 		inst.selectedDay = date.getDate();
@@ -1569,7 +1569,7 @@ $.extend( Datepicker.prototype, {
 				try {
 					return $.datepicker.parseDate( $.datepicker._get( inst, "dateFormat" ),
 						offset, $.datepicker._getFormatConfig( inst ) );
-				} catch ( e ) {
+				} catch ( _e ) {
 
 					// Ignore
 				}
diff --git a/ui/widgets/progressbar.js b/ui/widgets/progressbar.js
index 20e96440a..ad5366ade 100644
--- a/ui/widgets/progressbar.js
+++ b/ui/widgets/progressbar.js
@@ -9,9 +9,7 @@
 
 //>>label: Progressbar
 //>>group: Widgets
-/* eslint-disable max-len */
 //>>description: Displays a status indicator for loading state, standard percentage, and other progress indicators.
-/* eslint-enable max-len */
 //>>docs: https://api.jqueryui.com/progressbar/
 //>>demos: https://jqueryui.com/progressbar/
 //>>css.structure: ../../themes/base/core.css
diff --git a/ui/widgets/resizable.js b/ui/widgets/resizable.js
index 6f1e0ebde..012315283 100644
--- a/ui/widgets/resizable.js
+++ b/ui/widgets/resizable.js
@@ -104,7 +104,7 @@ $.widget( "ui.resizable", $.ui.mouse, {
 			el[ scroll ] = 1;
 			has = ( el[ scroll ] > 0 );
 			el[ scroll ] = 0;
-		} catch ( e ) {
+		} catch ( _e ) {
 
 			// `el` might be a string, then setting `scroll` will throw
 			// an error in strict mode; ignore it.
diff --git a/ui/widgets/selectmenu.js b/ui/widgets/selectmenu.js
index f1b48fa60..749e33491 100644
--- a/ui/widgets/selectmenu.js
+++ b/ui/widgets/selectmenu.js
@@ -9,9 +9,7 @@
 
 //>>label: Selectmenu
 //>>group: Widgets
-/* eslint-disable max-len */
 //>>description: Duplicates and extends the functionality of a native HTML select element, allowing it to be customizable in behavior and appearance far beyond the limitations of a native select.
-/* eslint-enable max-len */
 //>>docs: https://api.jqueryui.com/selectmenu/
 //>>demos: https://jqueryui.com/selectmenu/
 //>>css.structure: ../../themes/base/core.css
diff --git a/ui/widgets/tabs.js b/ui/widgets/tabs.js
index 7b7907c32..49468feb3 100644
--- a/ui/widgets/tabs.js
+++ b/ui/widgets/tabs.js
@@ -73,10 +73,10 @@ $.widget( "ui.tabs", {
 			// Decoding may throw an error if the URL isn't UTF-8 (#9518)
 			try {
 				anchorUrl = decodeURIComponent( anchorUrl );
-			} catch ( error ) {}
+			} catch ( _error ) {}
 			try {
 				locationUrl = decodeURIComponent( locationUrl );
-			} catch ( error ) {}
+			} catch ( _error ) {}
 
 			return anchor.hash.length > 1 && anchorUrl === locationUrl;
 		};